There is a growing demand in the mobile communication area for a system having the ability to download large blocks of data to a Mobile Station (MS) on demand at a reasonable rate. Such data could for example be web pages from the Internet, possibly including video clips or similar. Typically a particular MS will only require such data intermittently, so fixed bandwidth dedicated links are not appropriate. To meet this requirement in UMTS, a High-Speed Downlink Packet Access (HSDPA) scheme is being developed which may facilitate transfer of packet data to a mobile station at up to 4 Mbps.
A particular problem with the design of the HSDPA scheme is the mechanism for informing a MS of the presence of a data packet for it to receive and providing information relating to the packet (typically including details of the particular transmission scheme employed, for example spreading code, modulation scheme and coding scheme). As currently proposed, this information is signalled on one of four available downlink control channels, distinguished by their spreading codes. The MS is instructed to decode one of the control channels by a two-bit indicator signal which is transmitted on a low data rate dedicated downlink channel (the signal being inserted by puncturing). The MS then monitors the same control channel for subsequent packets in a burst.
This scheme conveniently supports the scheduling of up to four packets to different MSs in the same time interval. Use of the indicator signal is intended to reduce the complexity of the MS and its power consumption, as the MS only needs to monitor the dedicated downlink channel for the indicator signal instead of having to receive continuously all four control channels. However, there are significant drawbacks with the use of the indicator signal. One drawback is that an additional slot format is required for the dedicated downlink channel (to accommodate the extra signal), which adds complexity. Another drawback is that the transmission power required for the indicator signal can be relatively high to ensure reliable reception of the signal even at the edge of a cell.
One solution which avoids the use of an indicator signal is for each MS to be allocated one of the four control channels, which it then continuously monitors. However, if more than one MS is allocated the same control channel the flexibility of packet scheduling is restricted. Another solution is the provision of one control channel for each MS; however, the potentially large number of channels required could use up excessive system resources.